1. Field of the Invention
The present invention relates to marine anchors.
2. Description of the Prior Art
The fundamental requirement of a marine anchor is an ability to dig into a mooring bed when pulled forwardly, and to remain stable in the dug-in attitude in the bed when pulled further, it is also well established that for high holding power the anchor should be relatively deeply buried during anchor setting. The nature of mooring beds varies enormously, for example, from hard soils of granular noncohesive dense gravels and sands or cohesive stiff clays to soft soils of cohesive muds. The mooring bed may also be rocky whereupon the anchors must be able to hook satisfactorily onto a rock for mooring. Satisfactory operation of an anchor in a particular mooring bed has necessitated the anchor to have a particular geometry including a fluke angle compatible with the mooring bed soil. The fluke angle is the angle formed between the fluke and a line in a fore-and-aft plane of symmetry of the anchor extending between the rear of the fluke and an anchor line attachment point in the forward end of the shank. At present, it is known, (see, for example, The Quarterly Transactions of the Institute of Naval Architects, Vol. 92, No. 4 Oct. , 1950, pps. 341-343) that for operation in a sand bed a low fluke angle in the range 23.degree. to 32.degree. provides peak holding power in the deepest burying anchors. Fluke angles of 25.degree. to 32.degree. for medium dense to loose sands generally provide satisfactory performance. For a relatively soft mud bed, the fluke angle for peak performance is larger and is in the region of 50.degree. to 55.degree.. In sand with fluke angles over 32.degree., the moment about the anchor line attachment point of the resultant of soil normal pressure and friction forces on an anchor fluke is insufficient to counterbalance the sum of the moments about the same point of soil edge resistance force on the fluke and soil resistance force on the shank during initial penetration. The anchor is, in consequence, longitudinally unstable during pulling, and rotates about the attachment point into a nose-down attitude wherein it fails to bury below the surface of the mooring bed or even breaks out of the soil altogether. A fluke angle of 32.degree. or less has thus generally been adopted for the deepest burying anchors to permit effective use in both hard and soft soils. The resulting disadvantage in soft soils is usually mitigated by maximally increasing fluke area at the cost of reduced structural strength for hooking on rocks. However, even with increased fluke area, such anchors typically provide a soft mud performance less than 15 per cent of their sand performance. This illustrates the problem involved in providing an anchor with a single compromise fluke angle capable of producing high holding capacity in both hard sand and soft mud.
The applicant's European Patent No. 0180609 describes a marine anchor which, by the provision of a barrier plane aligned with transverse non-cohesive soil flow at the rear of the fluke and with a restriction passage between the barrier plate and the fluke, causes a stalled wedge of mud to accumulate on the fluke during burial in a soft mud bed. This mud wedge shears between the leading edges of the fluke and the upper edges of the barrier at an angle of 20.degree. to the fluke (which is set at a fluke angle of 30.degree. for sand) so that an effective fluke angle of 50.degree. is established at the incident-mud/stalled-mud-wedge interface. This large effective fluke angle at the surface of the stalled wedge enables the anchor to operate satisfactorily in soft mud. In a sand bed, the restriction passage, although too small to permit a significant through-flow of cohesive soil (mud), allows escape of non-cohesive soil (sand) aft from over the fluke whereby shearing occurs at the fluke surface so permitting effective operation of the anchor in sand at the actual fluke angle of 30.degree.. However, although this arrangement does provide improved capacity in mud, burial does not occur as deep as in the case of an anchor having a large fluke angle. Consequently, the very high holding capacity in soft mud of the deep based large fluke angle anchor is not achieved although the holding capacity does appreciably exceed that of the anchor with a small (sand) fluke angle When operating in mud. It is an object of the present invention to provide a marine anchor giving improved performance over the anchor of EP.0180609.
Another object of the present invention is to provide an improved marine anchor of the one-sided type (with the shank at one side only of the fluke) which self-orientates to a ground-engaging attitude when cast in an inverted position on and pulled horizontally over a mooring bed surface.
There can be problems in obtaining initial digging of an anchor in a hard clay bed, especially in the case of an anchor provided with means for self-orientating the anchor from an inverted position to a digging-in position, and it is a particular objective of the present invention to provide a marine anchor which obviates or mitigates this problem.